I'm right handed, and have started pushing the ball with my irons.
It's not my grip as that has not changed, and I don't think it's my stance either.
I am balanced and not moving my head. I don't get a push on my driver or fairway metal either.
What causes a push?

why? true or false the clubface is steadily closing during the downswing? if true then the clubface will be more open as it nears the ball being further centered in the stance, and an open clubface promotes a fade / slice.

Long, long ago (like 30 years ago) I used to pay attention to where the "V's" pointed,
but since then I use a technique much like Adam Scott where I hold the club in my left hand
and out to my left side with the toe of the club pointing towards the outside of my left foot.
I make sure that the heel line of the club is perpindicular to the outside line of my left foot, and
I just grab the golf grip with the top of my hand facing directly forward. Do this and you...

move the ball back more towards the center of your stance
choking down on the grip may be lessening the distance you had to the ball before and therefore as the clubface meets the ball your hands have already rotated and the clubface is not square but closed causing a draw or hook